1. Field of the Invention
The present invention relates to a power train having a multi-valve double overhead camshaft (DOHC) engine equipped with a mechanical supercharger and a transmission.
2. Description of Related Art
In order to increase the amount of air to be forced, under pressure, into an engine and thereby increase engine output torque, engines are typically equipped with superchargers. In addition, there have been known what are called Miller cycle engines in which closing of intake valves is retarded greatly after bottom dead center so as to make an effective compression ratio smaller than an expansion ratio, thereby reducing the work required for compression and reducing pumping loss. In recent years, it has been proposed to use this kind of method in engines equipped with superchargers and intercoolers with the aim of controlling knocking and boosting engine output torque. Such a Miller cycle engine is known, for instance, from Japanese Unexamined Utility Model Publication No. 63-51121.
The engine described in the above-mentioned publication has double overhead camshafts and is equipped with a supercharger and an intercooler provided in an intake passage. The time at which intake valves close is retarded later than 70.degree. after bottom dead center. With this engine, a reduction in effective compression ratio provides the effect of lowering the temperature at top dead center of the compression stroke so that knocking and temperature rise of exhaust gases are controlled. Under these controlled conditions, the supercharging volumetric efficiency is increased and engine output torque is effectively increased.
Engine output is transferred to wheels of the vehicle via a power transfer device, including a transmission and a reduction gear which are constituent elements of a power train, as well as an engine. Power trains installed in conventional passenger cars and the like, even if they include supercharged engines, have overall gear ratios of about 3.15 for the highest gears of the transmission, i.e., those having the smallest gear ratios. These power trains have maximum speeds of no less than 6,500 rpm. for a maximum engine output. In DOHC engines, in particular, because the DOHC mechanisms provide improved valve lifting performance at high speeds, the engines have increased maximum speeds so as to attain horsepower.
Conventional power trains of this kind achieve high engine output but provide poor fuel economy and have room for improvement in terms of noise, reliability and the like.